An effective modulation of the active sites in a bifunctional electrocatalyst is essentially desired, and it is a challenge to outperform the state-of-the-art catalysts toward oxygen electrocatalysis. Herein, we report the development of a bifunctional electrocatalyst having target-specific Fe–N4/C and Co–N4/C isolated active sites, exhibiting a symbiotic effect on overall oxygen electrocatalysis performances. The dualism of N-dopants and binary metals lower the d-band centers of both Fe and Co in the Fe,Co,N–C catalyst, improving the overpotential of the overall electrocatalytic processes (ΔE ORR‑OER = 0.74 ± 0.02 V vs RHE). Finally, the Fe,Co,N–C showed a high areal power density of 198.4 mW cm–2 and 158 mW cm–2 in the respective liquid and solid-state Zn–air batteries (ZABs), demonstrating suitable candidature of the active material as air cathode material in ZABs.

The rational design of electronically tuned transition-metal-doped conductive carbon nanostructures has emerged as a potential substitution of a platinum-group-metal (PGM)-free electrocatalyst for oxygen reduction reaction (ORR). We report here a universal strategy using a one-step thermal polymerization reaction for transition-metal-doped graphitic carbon nitride (g-C3N4) without any conductive carbon support as a highly efficient ORR electrocatalyst. X-ray absorption spectroscopy evidences the presence of Fe–Nx active sites with a possible three-coordinated Fe atom with N atoms. The as-prepared Fe-g-C3N4 with improved surface area, graphitic nature, and conductive carbon framework exhibits a superior electrochemical performance toward ORR activity in an alkaline medium. Interestingly, it displays a 0.88 V (vs reversible hydrogen electrode, RHE) half-wave potential (E 1/2) with a four-electron-transfer pathway and excellent stability outperforming platinum/carbon (Pt/C) in an alkaline medium. More impressively, when the Fe-g-C3N4 catalyst is used as a cathode material in a zinc–air battery, it presents a higher peak power density (148 mW cm–2) than Pt/C (133 mW cm–2), which further established the importance of the low-cost material synthesis approach toward the development of an earth-abundant PGM-free catalyst for fuel-cell and air battery fabrication.

Transition-metal atoms and/or heteroatom-doped carbon nanostructures is a crucial alternative to find a nonprecious metal catalyst for electrocatalytic oxygen reduction reaction (ORR). Herein, for the first time, we demonstrated the formation of binary (Fe–Mn) active sites in hierarchically porous nanostructure composed of Fe, Mn, and N-doped fish gill derived carbon (Fe,Mn,N-FGC). The Fe,Mn,N-FGC catalyst shows remarkable ORR performance with onset potential (E onset) of 1.03 V and half-wave potential (E 1/2) of 0.89 V, slightly better than commercial Pt/C (E onset = 1.01 V, E 1/2 = 0.88 V) in alkaline medium (pH > 13), which is attributed to the synergistic effect of Fe–Mn dual metal center as evidenced from X-ray absorption spectroscopic study. We proposed that the presence of Fe–Mn binary sites is actually beneficial for the O2 binding and boosting the ORR by weakening the OO bonds. The homemade rechargeable Zn–air battery performance reveals the open-circuit voltage of 1.41 V and a large power density of 220 mW cm–2 at 260 mA cm–2 current density outperforming Pt/C (1.40 V, 158 mW cm–2) with almost stable charge–discharge voltage plateaus at high current density. The present strategy enriches a route to synthesize low-cost bioinspired electrocatalyst that is comparable to/better than any nonprecious-metal catalysts as well as commercial Pt/C.

The growing demands for ammonia in agriculture and transportation fuel stimulate researchers to develop sustainable electrochemical methods to synthesize ammonia ambiently, to get past the energy-intensive Haber-Bosch process. However, the conventionally used aqueous electrolytes limit N 2 solubility, leading to insufficient reactant molecules in the vicinity of the catalyst during electrochemical nitrogen reduction reaction (NRR). This hampers the yield and production rate of ammonia, irrespective of how efficient the catalyst is. Herein, we introduce an aqueous electrolyte (NaBF 4 ), which not only acts as an N 2 -carrier in the medium but also works as a full-fledged “co-catalyst” along with our active material MnN 4 to deliver a high yield of NH 3 (328.59 μg h −1 mg cat −1 ) at 0.0 V versus reversible hydrogen electrode. BF 3 -induced charge polarization shifts the metal d-band center of the MnN 4 unit close to the Fermi level, inviting N 2 adsorption facilely. The Lewis acidity of the free BF 3 molecules further propagates their importance in polarizing the N≡N bond of the adsorbed N 2 and its first protonation. This push-pull kind of electronic interaction has been confirmed from the change in d-band center values of the MnN 4 site as well as charge density distribution over our active model units, which turned out to be effective enough to lower the energy barrier of the potential determining steps of NRR. Consequently, a high production rate of NH 3 (2.45 × 10 −9 mol s −1 cm −2 ) was achieved, approaching the industrial scale where the source of NH 3 was thoroughly studied and confirmed to be chiefly from the electrochemical reduction of the purged N 2 gas.

The interface engineering strategy has been an emerging field in terms of material improvisation that not only alters the electronic band structure of a material but also induces beneficial effects on electrochemical performances. Particularly, it is of immense importance for the environmentally benign electrochemical nitrogen reduction reaction (NRR), which is potentially impeded by the competing hydrogen evolution reaction (HER). The main problem lies in the attainment of the desired current density at a negotiable potential where the NRR would dominate over the HER, which in turn hampers the Faradaic efficiency for the NRR. To circumvent this issue, catalyst development becomes necessary, which would display a weak affinity for H-adsorption suppressing the HER at the catalyst surface. Herein, we have adopted the interfacial engineering strategy to synthesize our electrocatalyst NPG@SnS2, which not only suppressed the HER on the active site but yielded 49.3% F.E. for the NRR. Extensive experimental work and DFT calculations regarded that due to the charge redistribution, the Mott–Schottky effect, and the band bending of SnS2 across the contact layer at the interface of NPG, the d-band center for the surface Sn atoms in NPG@SnS2 lowered, which resulted in favored adsorption of N2 on the Sn active site. This phenomenon was driven even forward by the upshift of the Fermi level, and eventually, a decrease was seen in the work function of the heterostructure that increased the conductivity of the material as compared to pristine SnS2. This strategy thus provides a field to methodically suppress the HER in the realm of improving the Faradaic efficiency for the NRR.